Residual Strain Measurement in Cured Composite Parts

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Solution Overview

Problem

Existing methods for measuring residual strain in composite parts are limited by geometry restraints and spatial resolution, particularly when dealing with complex, high-gradient strain fields, and cannot provide full-field, multi-axial strain measurements without damaging the material or using multiple strain gages.

Innovation Solution

A method involving digital camera imaging to measure residual strain by taking initial and secondary images of particles on a cured composite part after removing a portion of one ply, allowing for full-field, high-resolution, multi-axial strain measurements without geometry limitations, using a camera assembly and abrading tools to relieve material restraints.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Measurement precision

If hole-drilling method is used to measure residual strain, then residual strain measurement is enabled, but geometry restraints and spatial resolution are limited

Engineering Contradiction:

Improvespatial resolutionVSAvoidgeometry restraints

Core Design Contradiction:

Measurement precisionVSAdaptability or versatility

Solution Approach 1:

The patent replaces the mechanical hole-drilling method with a non-contact optical measurement system using digital image correlation. Instead of physically removing material to measure strain, the system uses digital imaging to track particle displacements on the surface, thereby achieving high spatial resolution without geometry restraints.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

Solution Approach 2:

The patent creates a digital copy of the strain field by tracking the apparent displacement of particles captured in digital images. Rather than physically measuring strain through material removal, the system captures optical information and processes it computationally to determine strain measurements, enabling full-field measurement without physical interference.

Inventive Principle:

Principle #26Copying

2Measurement precision

If strain gages are used to measure residual strain, then strain measurement is enabled, but the occupation of space limits the measurement locations

Engineering Contradiction:

Improvespatial resolutionVSAvoidmeasurement coverage area

Core Design Contradiction:

Measurement precisionVSArea of stationary object

Solution Approach 1:

The patent creates a universal measurement system that can measure strain across the entire surface of a component simultaneously. The digital image correlation technique allows a single setup to capture full-field strain information across large areas, replacing the need for multiple discrete strain gages and enabling measurement of entire strain fields rather than isolated points.

Inventive Principle:

Principle #6Universality (Multi-functionality)

3Measurement precision

If thermal strain measurement method is used, then strain measurement is enabled, but only strain up to cure temperature is measured, not residual strain

Engineering Contradiction:

Improveresidual strain measurement accuracyVSAvoidresidual strain information

Core Design Contradiction:

Measurement precisionVSLoss of information

Solution Approach 1:

The patent applies preliminary action by first measuring the strain field in the cured component, then selectively removing material to release and measure the residual strain that was previously locked in. This two-step approach allows the system to capture both the initial strain state and the residual strain state, enabling accurate determination of residual strain through comparison.

Inventive Principle:

Principle #10Preliminary action

Applied Scientific Principles

This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.

Function Achieved in This Case

Enables direct measurement of residual strain in complex geometries with high spatial resolution and versatility, eliminating the need for strain gages and minimizing material damage, thereby improving the accuracy and applicability of strain measurements.

Implementation Method 1

taking a first image of at least two spaced apart particles positioned on a side of the cured composite part

Methodology Applied

Scientific EffectOptical imaging: Photography

Implementation Method 2

Residual strain is developed within a composite part with the elevation in temperature and the chemical reactions which the composite part experiences during the curing process

Methodology Applied

Scientific EffectThermal expansion: Thermal Expansion

Data Source

PatentEP3395571B1Method for measuring residual strain for cured composite part

Publication Date: 2021.04.21 THE BOEING CO

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AI summary

Method for measuring residual strain in a cured composite part includes taking a first image of at least two spaced apart particles positioned on a side of the cured composite part with the side positioned transverse to a first face side. The cure composite part includes a first ply has fibers and a second ply has fibers wherein the first ply and the second ply are positioned in overlying relationship to one another. At least a portion of the fibers of the first ply are positioned in angular relationship with at least a portion of the fibers of the second ply. The at least two spaced apart particles are associated with the second ply. Further included is a step of removing at least a portion of the first ply and taking a second image of the at least two spaced apart particles.